The present invention relates to a valve apparatus comprising a valve body and a valve box which have engagement portions formed with hard facing layers of cobalt-free Ni-base alloys and, more particularly, to a valve apparatus including engagement portions which have an excellent wear resistance.
In a conventional valve apparatus, for example, as disclosed in JP-A-62-1837, a combination of a Cr--Ni--Fe-system iron alloy and a Cr--Nb--Mo--Ni-system Ni-base alloy has been employed for hard facing layers provided on engagement portions of a valve body and engagement portions of a valve box. Overlays of these alloys, which are formed by overlay welding, tend to crack.
When hard facing layers on engagement portions of a valve body and engagement portions of a valve box are both formed of an Ni-base alloy, as disclosed in JP-A-62-130792, the hard facing layers on the valve body and the valve box have the same hardness, and consequently, the sliding wear property is supposedly inferior.
In JP-A-3-36088, a combination of an Ni-base alloy containing not more than 10% Fe and an Ni-base alloy containing 15% Fe is suggested to use for hard facing layers of a valve body and a valve box in a valve apparatus for a light-water nuclear reactor. The Ni-base alloys are different in hardness with each other and thus improved in wear resistance and erosion resistance.
Generally, in a valve apparatus disposed in a water pipe line for a light-water nuclear reactor, engagement portions and guide portions of a valve body and a valve box of the valve apparatus are formed with hard facing layers of Co-base alloys by overlay welding so as to provide the engagement portions and the guide portions with wear resistance and erosion resistance. However, the engagement portions and the guide portions of the valve body and the valve box are in contact with high-temperature/high-pressure water of the light-water nuclear reactor, and therefore, Co in the hard facing layers of Co-base alloys is dissolved into reactor water. The dissolved Co enters into the reactor core along with the reactor water and becomes Co60, and then, Co60 circulates and adheres to pipes, turbines and other devices. This is thought to be one of the reasons why the amount of radiation exposure to operators who perform valve overhauls tends not to decrease when a periodic check-up of a light-water nuclear reactor plant is conducted.
In view of the above, there has been known an example in which Ni-base alloys without Co are used as materials for overlay welding to be formed on the surfaces of engagement portions of a valve apparatus. However, the known combination of Ni-base alloys for the engagement portions of the valve apparatus is regarded as inferior in the sliding property, i.e., scoring resistance. More specifically, sliding-contact portions of a valve apparatus not only require wear resistance but also need to endure repetition of slidings and a high contact pressure. During sliding operation of a valve body especially having a large diameter, engagement portions of the valve body tend to deform when a difference between pressures upstream and downstream of the valve body is large, and consequently, local contact pressures of the engagement portions are increased. Thus, the known combination of Ni-base alloys has a possibility of deterioration of the sliding property.
A material of hard facing layers on engagement portions of a valve body and engagement portions of a valve box in common use is stellite (a Co-base alloy) which has an excellent sliding property. Therefore, a novel material of hard facing layers on engagement portions of a valve body and engagement portions of a valve box is required to have a sliding property as excellent as that of stellite.